docs: add note about default padding in crypto

Adds a note that the default padding for publicDecrypt/privateEncrypt
is RSA_PKCS1_PADDING instead of RSA_PKCS1_OAEP_PADDING as it is for
privateDecrypt/publicEncrypt.

PR-URL: #659
Reviewed-By: Brendan Ashworth <brendan.ashworth@me.com>

doc/api/crypto.markdown

@@ -710,10 +710,12 @@ NOTE: All paddings are defined in `constants` module.
 ## crypto.privateEncrypt(private_key, buffer)
 
 See above for details. Has the same API as `crypto.privateDecrypt`.
+Default padding is `RSA_PKCS1_PADDING`.
 
 ## crypto.publicDecrypt(public_key, buffer)
 
-See above for details. Has the same API as `crypto.publicEncrypt`.
+See above for details. Has the same API as `crypto.publicEncrypt`. Default
+padding is `RSA_PKCS1_PADDING`.
 
 ## crypto.DEFAULT_ENCODING